Zeolite material, a process of making such zeolite material, a product from such process, and the use thereof in the conversion of hydrocarbons

ABSTRACT

A process for making an improved zeolite catalyst composition comprising acid-treating a zeolite to provide an acid-treated zeolite, ion-exchanging the ions of such acid-treated zeolite with ions of zinc and at least one other metal in the presence of an ion-exchange medium, and then treating such acid-treated, ion-exchanged zeolite in a steam atmosphere. An improved zeolite catalyst composition made by such process is also disclosed. Processes are also disclosed for using the improved zeolite catalyst composition, made by the novel process, in the conversion of hydrocarbons, preferably non-aromatic hydrocarbons, to lower olefins (such as ethylene and propylene) and aromatic hydrocarbons (such as benzene, toluene, and xylene).

BACKGROUND OF THE INVENTION

The invention relates to an improved process for convertinghydrocarbons, preferably non-aromatic hydrocarbons, in the presence ofan improved zeolite material, to aromatic hydrocarbons and lower olefinhydrocarbons preferably with a low rate of coke formation during theconversion of such hydrocarbons in the presence of such improved zeolitematerial.

It is known to catalytically crack gasoline boiling range hydrocarbons(in particular, non-aromatic gasoline boiling range hydrocarbons, morein particular, paraffins and olefins) to lower olefins (such as ethyleneand propylene) and aromatic hydrocarbons (such as benzene, toluene, andxylene, and also ethylbenzene) in the presence of catalysts whichcontain a zeolite (such as ZSM-5), as is described in an article by N.Y. Chen et al. in Industrial & Engineering Chemistry Process Design andDevelopment, Volume 25, 1986, pages 151-155. The reaction product ofthis catalytic cracking process contains a multitude of hydrocarbonssuch as unconverted C₅ + alkanes, lower alkanes (methane, ethane,propane), lower alkenes (ethylene and propylene), C₆ -C₈ aromatichydrocarbons (e.g., benzene, toluene, xylene, and ethylbenzene), andC₉ + aromatic hydrocarbons. Depending upon the relative market prices ofthe individual reaction products, it can be desirable to increase theyield of certain of the more valuable products relative to the others.

One concern with the use of zeolite catalysts in the conversion ofhydrocarbons to aromatic hydrocarbons and lower olefins is the excessiveproduction of coke during the conversion reaction. The term "coke"refers to a semi-pure carbon generally deposited on the metal surfacesof process equipment or a catalyst. Coke formed during the zeolitecatalyzed aromatization of hydrocarbons tends to cause catalystdeactivation. It is desirable to improve processes for the aromatizationof hydrocarbons, and the formation of lower olefins from hydrocarbons,by minimizing the amount of coke formed during such processes. It isalso desirable to have a zeolite catalyst that is useful in producingsignificant quantities of the aromatic and olefin conversion products.

SUMMARY OF THE INVENTION

It is an object of this invention to provide a zeolite catalystcomposition used to at least partially convert hydrocarbons to lowerolefins (such as ethylene and propylene) and aromatic hydrocarbons (suchas benzene, toluene, xylene and ethylbenzene, i.e., BTX).

A further object of this invention is to provide an improved process forthe conversion of hydrocarbons in which the rate of coke formationduring such conversion of hydrocarbons is minimized.

A yet further object of this invention is to provide an improved zeolitematerial which, when used in the conversion of hydrocarbons, results inless coke formation than alternative zeolite materials.

Another object of this invention is to provide an improved zeolitematerial that gives an improved yield of lower olefins when suchimproved zeolite material is utilized in the conversion of hydrocarbons.

Yet another object of this invention is to provide hydrocarbonconversion processes which have an acceptably low coke production rateand/or which produce a conversion product containing suitable quantitiesof lower olefins and BTX aromatics.

Yet another further object of this invention is to provide a method formaking an improved zeolite material having such desirable properties asproviding for low coke production and improved yields of lower olefins,with an especially improved ratio of lower olefins to aromatics in theproduct, when used in the conversion of hydrocarbons.

One embodiment of the invention is a novel process of making a zeolitecatalyst composition used in the conversion of hydrocarbons, preferablynon-aromatic hydrocarbons, to aromatic hydrocarbons and lower olefins.The novel process comprises ion-exchanging the original ions(specifically cations) such as, for example, alkali metal ions oralkaline earth metal ions, of a zeolite with hydrogen ions byacid-treating such zeolite. The cations, preferably hydrogen ions, ofsuch acid-treated zeolite are then further ion-exchanged with ions ofzinc and at least one other metal selected from the group of metalsconsisting of Group 6B of the periodic table of elements to therebyprovide an acid-treated, ion-exchanged zeolite. The ion-exchange of suchacid-treated zeolite occurs in the presence of an ion-exchange medium,preferably comprising an aqueous solution of an ammonium-containingcompound, a zinc-containing compound, and a compound containing at leastone other metal, which promotes the exchange of ions of the acid-treatedzeolite with ions of zinc and at least one other metal. Theacid-treated, ion-exchanged zeolite is then subjected to a steamtreatment to provide the final improved zeolite catalyst composition.

Another embodiment of the invention is a process for the conversion ofnon-aromatic hydrocarbons to aromatic hydrocarbons and lower olefins bycontacting, under conversion conditions, a hydrocarbon-containing fluidwith an acid-treated, ion-exchanged, steam-treated zeolite catalystcomposition.

Yet another embodiment of the invention is the novel composition of anacid-treated zeolite of which the ions of such zeolite have beenion-exchanged with ions of zinc and at least one other metal selectedfrom the group of metals consisting of Group 6B of the periodic table ofelements. The acid-treated, ion-exchanged zeolite is then subjected to asteam treatment to provide the final improved zeolite catalystcomposition.

Yet another embodiment of the invention is the novel composition, i.e.,product, made by the novel process of ion-exchanging the original ions(specifically cations) such as, for example, alkali metal ions oralkaline earth metal ions, of a zeolite with ions of hydrogen byacid-treating such zeolite. The cations, preferably hydrogen ions, ofsuch acid-treated zeolite are then further ion-exchanged with ions ofzinc and at least one other metal from the group of metals consisting ofGroup 6B of the periodic table of elements. The ion-exchange of suchacid-treated zeolite occurs in the presence of an ion-exchange medium,preferably comprising an aqueous solution of an ammonium-containingcompound, a zinc-containing compound, and a compound containing at leastone other metal, which promotes the exchange of ions of the acid-treatedzeolite with ions of zinc and at least one other metal. Theacid-treated, ion-exchanged zeolite is then subjected to a steamtreatment to provide the final improved zeolite catalyst composition.

Other objects and advantages of the invention will become apparent fromthe detailed description and the appended claims.

DETAILED DESCRIPTION OF THE INVENTION

The inventive composition includes a zeolite starting material that hasbeen ion-exchanged such that a predominant proportion of such zeolite'sexchangeable ions (specifically cations) are hydrogen (H⁺) ions.Preferably, such zeolite starting material has been treated with an acidto thereby provide an acid-treated zeolite in which a predominantproportion of such acid-treated zeolite's exchangeable ions(specifically cations) are hydrogen (H⁺) ions. In general, it iscontemplated that more than 50 percent and preferably more than 75percent of the cationic sites of such acid-treated zeolite will beoccupied by hydrogen ions. After further ion-exchange of suchacid-treated zeolite, the resulting inventive composition furthercontains ions of zinc and at least one other metal or element selectedfrom the group of elements consisting of Group 6B of the periodic tableof elements. It is understood herein that any reference to at least oneother metal in addition to zinc contained in the inventive compositionwill be an element from the Group 6B elements including Chromium (Cr),Molybdenum (Mo), and Tungsten (W). As the term is used within thisdescription and in the claims, any reference to metals will include zincand those Group 6B elements listed above.

An important aspect of the invention is the requirement that theoriginal ions (specifically cations), such as, for example, alkali metalions or alkaline earth metal ions, of a zeolite preferably beion-exchanged with hydrogen ions (H⁺) by acid-treating such zeolite toprovide an acid-treated zeolite. While less preferred, the original ionsof the zeolite may be ion-exchanged with hydrogen ions (H⁺) via initialammonium exchange followed by calcination.

The cations, preferably hydrogen ions, of such acid-treated zeolite arethen dual ion-exchanged, i.e., simultaneously ion-exchanged with ions ofzinc and at least one other metal in the presence of an ion-exchangemedium, preferably comprising an aqueous solution of anammonium-containing compound, a zinc-containing compound, and a compoundcontaining at least one other metal, which promotes the exchange of ionsof the acid-treated zeolite with ions of zinc and at least one othermetal. The dual or simultaneous ion-exchange of such acid-treatedzeolite in the presence of such ion-exchange medium creates anequilibrium (i.e., competitional) ion-exchange environment in which theammonium ions (NH₄ ⁺) of the ammonium-containing compound attempt toexchange with the cations, preferably hydrogen ions, of the acid-treatedzeolite. However, the ammonium ions must "compete" with the ions of zincand at least one other metal to ion-exchange (hence the term"competitional ion-exchange") with the cations of the acid-treatedzeolite. The competitional ion-exchange environment allows for a betterdispersion of the ions of zinc and at least one other metal within theacid-treated zeolite.

A yet further important aspect of the novel process of making thecatalyst is a steam-treating step. The steam-treating step includes asteam treatment of the acid-treated, ion-exchanged zeolite subsequent tosuch ion-exchange, as described above, of the original ions of a zeolitewith ions of zinc and at least one other metal. The use of thesteam-treatment step produces an acid-treated, ion-exchanged,steam-treated zeolite catalyst composition containing ions of zinc andat least one other metal that provides an improved lower olefin yieldand an improved (i.e., greater) olefins-to-aromatics ratio when used inthe conversion of hydrocarbons, preferably non-aromatic hydrocarbons,than a catalyst made by certain methods other than the inventive methoddescribed herein.

To make the improved zeolite catalyst having been ion-exchanged withions of zinc and at least one other metal, a starting zeolite or zeolitematerial is, preferably, first treated with an acid to form anacid-treated zeolite in which the original ions (specifically cations)such as, for example, alkali metal ions or alkaline earth metal ions, ofthe starting zeolite or zeolite material are ion-exchanged with hydrogenions (H⁺). Methods known to one skilled in the art can be used toion-exchange the zeolite starting material with hydrogen ions such asthose disclosed in U.S. Pat. No. 5,516,956, the disclosure of which isincorporated herein by reference.

The zeolite starting material used in the composition of the inventioncan be any zeolite which is effective in the conversion of hydrocarbons,preferably non-aromatic hydrocarbons, to lower olefin hydrocarbons andaromatic hydrocarbons when contacted under suitable reaction conditions.Examples of suitable zeolites include, but are not limited to, thosedisclosed in Kirk-Othmer Encyclopedia of Chemical Technology, thirdedition, volume 15, pages 638-669 (John Wiley & Sons, New York, 1981).Preferably, the zeolite has a constraint index (as defined in U.S. Pat.No. 4,097,367, which is incorporated herein by reference) in the rangeof from about 0.4 to about 12, preferably in the range of from about 2to about 9. Generally, the molar ratio of SiO₂ to Al₂ O₃ in thecrystalline framework of the zeolite is at least about 5:1 and can rangeup to infinity. Preferably the molar ratio of SiO₂ to Al₂ O₃ in thezeolite framework is in the range of from about 8:1 to about 200:1, morepreferably in the range of from about 12:1 to about 100:1. Preferredzeolites include ZSM-5, ZSM-8, ZSM-11, ZSM-12, ZSM-35, ZSM-38, andcombinations thereof. Some of these zeolites are also known as "MFI" or"Pentasil" zeolites. The presently more preferred zeolite is ZSM-5.

To produce a zeolite in the hydrogen-exchanged form, the zeolitestarting material is, preferably, treated with an acid by any suitablemeans or method(s) that result in an acid-treated zeolite. Generally,any organic acid, inorganic acid, or combinations thereof can be used inthe process of the present invention so long as the acid provides asource of hydrogen ions for exchange with the original ions(specifically cations), such as, for example, alkali metal ions oralkali earth metal ions, of the zeolite. The acid can also be a dilutedaqueous acid solution. Examples of possible acids include, but are notlimited to, sulfuric acid, hydrochloric acid, nitric acid, phosphoricacid, formic acid, acetic acid, trifluoroacetic acid, trichloroaceticacid, p-toluenesulfonic acid, methanesulfonic acid, partiallyneutralized acids wherein one or more protons have been replaced with,for example, a metal (preferably an alkali metal), and combinationsthereof. Examples of partially neutralized acids include, but are notlimited to, sodium bisulfate, sodium dihydrogen phosphate, potassiumhydrogen tartarate, ammonium sulfate, ammonium chloride, ammoniumnitrate, and combinations thereof. The presently preferred acid isaqueous hydrochloric acid.

Any method(s) known to one skilled in the art for treating a solidcatalyst with an acid can be used in the acid treatment of the presentinvention. Generally, a zeolite material can be suspended in an acidsolution. The concentration of the zeolite in the acid solution can bein the range of from about 0.01 gram per liter to about 500 grams perliter, preferably in the range of from about 0.1 gram per liter to about400 grams per liter, more preferably in the range of from about 1 gramper liter to about 350 grams per liter, and most preferably in the rangefrom 5 grams per liter to 300 grams per liter. The amount of acidrequired is the amount that can maintain the solution in acidic pH(i.e., pH less than about 7) during the treatment. Preferably, theinitial pH of the acid solution containing a zeolite is adjusted tolower than about 6, preferably lower than about 5, more preferably lowerthan about 4, and most preferably lower than 3.

Upon the pH adjustment of the solution, the solution can be subjected toa treatment at a temperature in the range of from about 30° C. to about200° C., preferably in the range of from about 50° C. to about 150° C.,and most preferably in the range from 70° C. to 120° C. for a timeperiod in the range of from about 1 minute to about 30 hours, preferablyin the range of from about 5 minutes to about 25 hours, and mostpreferably in the range from 10 minutes to 20 hours. The treatment canbe carried out under a pressure in the range of from about atmosphericto about 150 pounds per square inch absolute (psia), preferably aboutatmospheric, so long as the desired temperature can be maintained.

Thereafter, the acid-treated zeolite material can be washed with runningwater for a time period in the range of from about 1 minute to about 60minutes followed by drying, at a temperature in the range of from about50° C. to about 100° C., preferably in the range of from about 75° C. toabout 750° C., and most preferably in the range from 100° C. to 650° C.for a time period in the range of from about 0.5 hour to about 15 hours,preferably in the range of from about 1 hour to about 12 hours, and mostpreferably in the range from 1 hour to 10 hours, to produce anacid-treated zeolite. Any drying method(s) known to one skilled in theart such as, for example, air drying, heat drying, spray drying,fluidized bed drying, or combinations thereof can be used.

The dried, acid-treated zeolite can also be further washed, if desired,with a mild acid solution such as, for example, ammonium nitrate whichis capable of maintaining the pH of the wash solution in acidic range(i.e., a pH of less than about 7). The volume of the acid generally canbe the same volume as the acid for reducing the alumina content in azeolite. The mild acid treatment can be carried out under substantiallythe same conditions as disclosed above for the preparation of anacid-treated zeolite. Thereafter, the resulting solid can be washed anddried as disclosed above.

The dried, acid-treated zeolite, whether it has been further washed witha mild acid or not, can be calcined, if desired, under conditions knownto those skilled in the art. Generally, such conditions can include atemperature in the range of from about 250° C. to about 1,000° C.,preferably in the range of from about 350° C. to about 750° C., and mostpreferably in the range from 450° C. to 650° C. and a pressure in therange of from about 7 pounds per square inch absolute (psia) to about750 psia, preferably in the range of from about 7 psia to about 450psia, and most preferably in the range from 7 psia to 150 psia for atime period in the range of from about 1 hour to about 30 hours,preferably in the range of from about 2 hours to about 20 hours, andmost preferably in the range from 3 hours to 15 hours.

The acid-treated zeolite is then treated in an ion-exchange mediumselected from the group consisting of water, organic solvents, andcombinations thereof. Ion-exchange medium refers to any medium thatpermits the ion-exchange of such acid-treated zeolite. Typical organicsolvents include alcohols, esters, ethers, ketones, and the like andcombinations thereof. The preferred ion-exchange medium is water.

The ion-exchange medium, preferably water, further comprises anammonium-containing compound, a zinc-containing compound and a compoundcontaining at least one other metal. An ammonium-containing compoundrefers to a compound containing an exchangeable ammonium ion, forexample NH₄ ⁺. A zinc-containing compound refers to a compoundcontaining an exchangeable zinc ion, for example Zn⁺². A compoundcontaining at least one other metal refers to a compound containing anexchangeable ion of at least one other metal, for example Cr⁺³.Preferably, the acid-treated zeolite is treated in an ion-exchangemedium comprising an aqueous solution of an ammonium-containingcompound, a zinc-containing compound, and a compound containing at leastone other metal.

Examples of suitable ammonium-containing compounds include, but are notlimited to, ammonium nitrate, ammonium sulfate, ammonium chloride,ammonium bromide, ammonium fluoride, and combinations thereof. Thepreferred ammonium-containing compound is ammonium nitrate. Treatment ofthe zeolite in an ion-exchange medium comprising an aqueous solution ofan ammonium-containing compound, a zinc-containing compound, and acompound containing at least one other metal creates an environment inwhich the ammonium ions compete, with the ions of zinc and at least oneother metal, to exchange with the cations, preferably hydrogen ions, ofthe acid-treated zeolite.

A novel, yet not fully understood, aspect of this invention is thereaction mechanism(s) in which the cations, preferably hydrogen ions, ofthe acid-treated zeolite are ion-exchanged with ions of zinc and atleast one other metal. Wishing not to bound by any theory, one possiblereaction mechanism is that the cations of the acid-treated zeolite mayinitially ion-exchange with ammonium ions of which such ammonium ionsare further ion-exchanged with ions of zinc and at least one othermetal. Another possible reaction mechanism is that the cations of theacid-treated zeolite may be simultaneously ion-exchanged with ions ofammonium, zinc, and at least one other metal. Yet another possiblereaction mechanism is that the higher ion-binding forces of zinc and atleast one other metal hinder the ammonium ions from exchanging with thecations of the acid-treated zeolite, allowing mostly ions of zinc and atleast one other metal to exchange with the cations of the acid-treatedzeolite. Any of these possible reaction mechanisms may be occurring andmay even be occurring simultaneously.

Generally, an acid-treated zeolite can be suspended in a solution,preferably an aqueous solution, of an ammonium-containing compound, azinc-containing compound, and a compound containing at least one othermetal. The concentration of the acid-treated zeolite in such aqueoussolution can be in the range of from about 0.01 gram of acid-treatedzeolite per liter of aqueous solution (gm/L) to about 200 gm/L,preferably in the range of from about 0.1 gm/L to about 150 gm/L, morepreferably in the range of from about 1 gm/L to about 100 gm/L, and mostpreferably in the range from 5 gm/L to 75 gm/L.

The amount of an ammonium-containing compound, a zinc-containingcompound, and a compound containing at least one other metal in theaqueous solution depends on the amount of the original ion(s) to beexchanged. Generally, the concentration of an ammonium-containingcompound in the aqueous solution can be in the range of from about 0.1gram of ammonium-containing compound per liter of aqueous solution(gm/L) to about 500 gm/L, preferably in the range of from about 10 gm/Lto about 400 gm/L, more preferably in the range of from about 20 gm/L toabout 300 gm/L, and most preferably in the range from 50 gm/L to 200gm/L.

Generally, the concentration of a zinc-containing compound in theaqueous solution can be in the range of from about 0.1 gram ofzinc-containing compound per liter of aqueous solution (gm/L) to about500 gm/L, preferably in the range of from about 1 gm/L to about 400gm/L. more preferably in the range of from about 10 gm/L to about 300gm/L, and most preferably in the range from 20 gm/L to 200 gm/L.

Generally, the concentration of a compound containing at least one othermetal in the aqueous solution can be in the range of from about 0.1 gramof compound containing at least one other metal per liter of aqueoussolution (gm/L) to about 500 gm/L, preferably in the range of from about1 gm/L to about 400 gm/L, more preferably in the range of from about 10gm/L to about 300 gm/L, and most preferably in the range from 20 gm/L to200

Upon the preparation of an acid-treated zeolite suspended in a solution,preferably aqueous solution, of an ammonium-containing compound, azinc-containing compound, and a compound containing at least one othermetal, the solution can be subjected to a temperature in the range offrom about 30° C. to about 200° C., preferably in the range of fromabout 40° C. to about 150° C., and most preferably in the range from 50°C. to 125° C. for a time period in the range of from about 1 hour toabout 100 hours, preferably in the range of from about 1 hour to about50 hours, and most preferably in the range from 2 hours to 25 hours,depending on desired degrees of ion exchange, and under a pressure inthe range of from about atmospheric to about 150 pounds per square inchabsolute (psia), preferably in the range of from about atmospheric toabout 80 psia, or any pressure that can maintain the requiredtemperature. Thereafter, the acid-treated, ion-exchanged zeolite can bewashed with running water for a time period in the range of from about 1minute to about 60 minutes followed by drying and calcining to producean acid-treated, ion-exchanged, calcined zeolite. The drying andcalcining processes can be carried out substantially the same as thosedisclosed above for the preparation of an acid-treated zeolite.

The acid-treated, ion-exchanged zeolite is then subjected to a steamtreatment whereby it is exposed by any suitable means or method(s) knownin the art to an atmosphere of steam under process conditions thatsuitably provide an acid-treated, ion-exchanged, steam-treated zeolite.The acid-treated, ion-exchanged zeolite is exposed to a predominantlygaseous atmosphere, preferably an entirely gaseous atmosphere,comprising steam. The steam atmosphere preferably has a concentration ofsteam exceeding about 90 molar percent and, most preferably, theconcentration of the steam atmosphere exceeds about 95 molar percent.

The steam treatment may be conducted at any pressure and temperatureconditions that suitably provide the acid-treated, ion-exchanged,steam-treated zeolite. Generally, the steam treatment may be conductedat a pressure in the range of from below atmospheric upwardly to about1000 pounds per square inch absolute (psia). More typical pressures,however, are in the range of from about atmospheric to about 100 psia.The steam treatment temperature is generally in the range of from about100° C. to about 1000° C. Preferably, this temperature is in the rangeof from about 101° C. to about 800° C. and, most preferably, the steamtreatment temperature is in the range from 102° C. to 700° C.

The time period for conducting the steam treatment step must besufficient to provide an acid-treated, ion-exchanged, steam-treatedzeolite suitable for providing a zeolite catalyst composition havingdesired properties such as low coke formation and improved lower olefinyield. Generally, the time period for exposing the acid-treated,ion-exchanged zeolite to the atmosphere of steam at appropriatetemperature conditions can be in the range of from about 0.1 hour toabout 30 hours. Preferably, the steam treatment step is conducted for atime period in the range of from about 0.25 hour to about 25 hours and,most preferably, in the range from 0.5 hour to 20 hours.

Examples of a potentially suitable zinc-containing compound for use inion-exchanging the ions of the acid-treated zeolite with zinc ionsinclude, but are not limited to, zinc nitrate, hydrated zinc nitrate,zinc acetate dehydrate, zinc acetylacetonate hydrate, zinc bromide, zinccarbonate hydroxide, zinc chloride, zinc cyclohexanebutyrate dihydrate,zinc 2-ethylhexanoate, zinc 2-ethylhexanoate, zinc fluoride, zincfluoride tetrahydrate, zinc hexafluoroacetylacetonate dihydrate, zinciodide, zinc molybdate, zinc naphthenate, zinc nitrate hexahydrate, zincperchlorate hexahydrate, zinc phosphate hydrate, zinc phthalocynine,zinc protoporphyrin, zinc selenide, zinc sulfate monohydrate, zincsulfide, zinc telluride, zinc tetrafluoroborate hydrate, zincmeso-tetraphenylprophine, zinc titanate, zinc trifluoromethanesulfonate,and combinations thereof.

The preferred zinc-containing compound is zinc nitrate, more preferablyhydrated zinc nitrate, and most preferably zinc nitrate hexahydrate asthese zinc-containing compounds are readily available and effective forion-exchange of the zinc ions of the zinc-containing compound with theions of the acid-treated zeolite.

The at least one other metal for use in ion-exchange with theacid-treated zeolite can be any Group 6B metal-containing compound thatcan promote the ion-exchange of the metal ions of the metal-containingcompound with the ions of the acid-treated zeolite.

Examples of suitable chromium-containing compounds include, but are notlimited to, chromium(II) acetate, chromium(III) acetate, chromium(III)acetylacetonate, chromium(II) chloride, chromium(III) chloridechromium(II) fluoride, chromium(III) fluoride, chromium(III) nitrate,hydrated chromium (III) nitrate, chromium (III) nitrate nonahydrate,chromium nitride, chromium(III) perchlorate, chromium(III) potassiumsulfate, chromium(III) sulfate, chromium(III) telluride, andcombinations thereof.

Examples of suitable molybdenum-containing compounds include, but arenot limited to, molybdenum(II) acetate, ammonium molybdate, ammoniumdimolybdate, ammonium heptamolybdate, phosphomolybdic acid,molybdenum(III) bromide, molybdenum(II) chloride, molybdenum(IV)chloride, molybdenum(V) chloride, molybdenum(IV) sulfide, sodiummolybdate, potassium molybdate, molybdenum fluoride, and combinationsthereof.

Examples of suitable tungsten-containing compounds include, but are notlimited to, tungsten(V) bromide, tungsten(IV) chloride, tungsten(VI)chloride, tungsten(IV) sulfide, tungstic acid, and combinations thereof.

The preferred metal-containing compound is chromium (III) nitrate, morepreferably hydrated chromium (III) nitrate, and most preferably chromium(III) nitrate nonahydrate as these metal-containing compounds arereadily available and effective for ion-exchange of the chromium ions ofthe metal-containing compound with the ions of the acid-treated zeolite.

The amounts of zinc and at least one other metal ion-exchanged with theacid-treated zeolite should be such as to give concentrations, of suchmetals in the final improved zeolite catalyst composition, effective inproviding the desirable properties of favorable (i.e., greater) olefinconversion yields, favorable (i.e., greater) olefins-to-aromatics ratio,and low coke production when the improved zeolite catalyst composition,as manufactured by the process described herein, is employed in theconversion of hydrocarbons, preferably non-aromatic hydrocarbons.

Generally, the amount of zinc and at least one other metal ion-exchangedwith the acid-treated zeolite is such that the atomic ratio of the atleast one other metal to zinc in the final improved zeolite catalystcomposition is in the range of from about 0.1:1 to about 10:1. Apreferred atomic ratio of the at least one other metal to zinc in thefinal improved zeolite catalyst composition is in the range of fromabout 0.2:1 to about 6:1 and, most preferably, the atomic ratio of theat least one other metal to zinc is in the range from 0.5:1 to 5:1.

Generally, the amount of zinc ion-exchanged with the acid-treatedzeolite is such that the weight percent of zinc present in the finalimproved zeolite catalyst composition is generally in the range upwardlyto about 10 weight percent of the total weight of the final improvedzeolite catalyst composition. The preferred concentration of the zinccomponent in the final improved zeolite catalyst composition is in therange of from about 0.1 weight percent of the total weight of the finalimproved zeolite catalyst composition to about 10 weight percent of thetotal weight of the final improved zeolite catalyst composition and,most preferably, in the range from 0.5 weight percent of the totalweight of the final improved zeolite catalyst composition to 5 weightpercent of the total weight of the final improved zeolite catalystcomposition.

Generally, the amount of the at least one other metal ion-exchanged withthe acid-treated zeolite is such that the weight percent of the at leastone other metal present in the final improved zeolite catalystcomposition is generally in the range upwardly to about 10 weightpercent of the total weight of the final improved zeolite catalystcomposition. The preferred concentration of the at least one other metalin the final improved zeolite catalyst composition is in the range offrom about 0.1 weight percent of the total weight of the final improvedzeolite catalyst composition to about 10 weight percent of the totalweight of the final improved zeolite catalyst composition and, mostpreferably, in the range from 0.5 weight percent of the total weight ofthe final improved zeolite catalyst composition to 5 weight percent ofthe total weight of the final improved zeolite catalyst composition.

The improved zeolite catalyst composition described herein can alsocontain an inorganic binder (also called matrix material) preferablyselected from the group consisting of alumina, silica, alumina-silica,aluminum phosphate, clays (such as bentonite), and combinations thereof.The content of the zeolite component (e.g., acid-treated zeolite,acid-treated, ion-exchanged zeolite, or acid-treated, ion-exchanged,steam-treated zeolite) of the optional mixture, of zeolite component andinorganic binder, is in the range of from about 1 weight percent of thetotal weight of the optional mixture to about 99 weight percent of thetotal weight of the optional mixture. Preferably, the content of thezeolite component of the optional mixture is in the range of from about5 weight percent of the total weight of the optional mixture to about 80weight percent of the total weight of the optional mixture.

Any suitable means for mixing the zeolite component and binder can beused to achieve the desired dispersion of the materials in the resultingadmixture. Many of the possible mixing means suitable for use inpreparing the mixture of zeolite component and binder of the inventivemethod are described in detail in Perry's Chemical Engineers' Handbook;Sixth Edition, published by McGraw-Hill, Inc., copyright 1984, at pages21-3 through 21-10, which pages are incorporated herein by reference.Thus, suitable mixing means can include, but are not limited to, suchdevices as tumblers, stationary shells or troughs, Muller mixers, whichare either batch type or continuous type, impact mixers, and the like.

It can be desirable to form an agglomerate of the mixture of zeolitecomponent and binder. Any suitable means known by those skilled in theart for forming such an agglomerate can be used. Such methods include,for example, molding, tableting, pressing, pelletizing, extruding,tumbling, and densifying. Further discussion of such methods is providedin a section entitled "Size Enlargement" in Perry's Chemical Engineers'Handbook, Sixth Edition, published by McGraw-Hill, Inc., copyright 1984,at pages 8-60 through 8-72, which pages are incorporated herein byreference.

Generally, the zeolite and inorganic binder components are compoundedand subsequently shaped (such as by pelletizing, extruding or tableting)into a compounded composition. Generally, the surface area of thecompounded composition is in the range of from about 50 m² /g to about700 m² /g. Generally, the particle size of the compounded composition isin the range of from about 1 mm to about 10 mm.

Any suitable hydrocarbon-containing fluid which comprises paraffins(alkanes) and/or olefins (alkenes) and/or naphthenes (cycloalkanes),wherein each of these hydrocarbons contains in the range of from about 2carbon atoms per molecule to about 16 carbon atoms per molecule, can beused as the fluid to be contacted with the improved zeolite catalystcomposition under suitable process conditions for obtaining a reactionproduct comprising lower olefins (alkenes, such as ethylene andpropylene) containing in the range of from about 2 carbon atoms permolecule to about 5 carbon atoms per molecule and aromatic hydrocarbons(such as BTX, i.e., benzene, toluene, and xylene). Frequently, thesuitable hydrocarbon-containing fluid also contains aromatichydrocarbons. The term "fluid" is used herein to denote gas, liquid,vapor, or combinations thereof.

Non-limiting examples of suitable, available hydrocarbon-containingfluid include gasolines from catalytic oil cracking (e.g., FCC andhydrocracking) processes, pyrolysis gasolines from thermal hydrocarbon-(e.g., ethane, propane, and naphtha) cracking processes, naphthas, gasoils, reformates, straight-run gasoline and combinations thereof. Thoughthe particular composition of the fluid is not critical, the preferredhydrocarbon-containing fluid is a gasoline-boiling rangehydrocarbon-containing fluid suitable for use as at least a gasolineblend stock generally having a boiling range of about 30° C. to about210° C. Generally, the content of paraffins exceeds the combined contentof olefins, naphthenes and aromatics (if present).

The hydrocarbon-containing fluid can be contacted by any suitable means,method(s), or manner with the improved zeolite catalyst composition,described herein, contained within a reaction zone. i.e., conversionzone. The contacting step can be operated as a batch process step or,preferably, as a continuous process step. In the latter operation, asolid catalyst bed, or a moving catalyst bed, or a fluidized catalystbed can be employed. Any of these operational modes have advantages anddisadvantages, and those skilled in the art can select the one mostsuitable for a particular fluid and catalyst.

The contacting step is preferably carried out within a conversion zone,wherein is contained the improved zeolite catalyst composition, andunder reaction conditions, i.e., conversion conditions, that suitablypromote the formation of olefins, preferably lower olefins (i.e., lightolefins such as ethylene and propylene), and aromatics, preferably BTXfrom at least a portion of the hydrocarbons of thehydrocarbon-containing fluid. Thus, the reaction product, i.e., theconversion product, includes olefins and aromatics.

Reaction, or conversion, conditions would include a reaction temperatureof the contacting step preferably in the range of from about 400° C. toabout 800° C., more preferably in the range of from about 450° C. toabout 750° C. and, most preferably in the range from 500° C. to 700° C.The contacting pressure can be in the range of from below atmosphericpressure upwardly to about 500 pounds per square inch absolute (psia),preferably, from about atmospheric to about 450 psia and, mostpreferably, from 20 psia to 400 psia.

The flow rate at which the hydrocarbon-containing fluid is charged(i.e., the charge rate of hydrocarbon-containing fluid) to theconversion zone is such as to provide a weight hourly space velocity("WHSV") in the range of from exceeding 0 hour⁻¹ upwardly to about 1000hour⁻¹. The term "weight hourly space velocity", as used herein, shallmean the numerical ratio of the rate at which a hydrocarbon-containingfluid is charged to the conversion zone in pounds per hour divided bythe pounds of catalyst contained in the conversion zone to which thehydrocarbon-containing fluid is charged. The preferred WHSV of thehydrocarbon-containing fluid to the conversion to zone can be in therange of from about 0.25 hour⁻¹ to about 250 hour⁻¹ and, most preferablyin the range from 0.5 hour⁻¹ to 100 hour⁻¹.

The process effluent from the conversion zone generally contains: alight gas fraction comprising hydrogen and methane, a C₂ -C₃ fractioncontaining ethylene, propylene, ethane, and propane, an intermediatefraction including non-aromatic compounds having greater than 3 carbonatoms, a BTX aromatic hydrocarbons fraction (benzene, toluene,ortho-xylene, meta-xylene and para-xylene), and a C₉ + fraction whichcontains aromatic compounds having 9 or more carbon atoms per molecule.

Generally, the process effluent can be separated into these principalfractions by any known method(s) such as, for example, fractionationdistillation. Because the separation method(s) are well known to oneskilled in the art, the description of such separation method(s) isomitted herein. The intermediate fraction can be fed to an aromatizationreactor to be converted to aromatic hydrocarbons. The methane, ethane,and propane can be used as fuel gas or as a feed for other reactionssuch as, for example, in a thermal cracking process to produce ethyleneand propylene. The olefins can be recovered and further separated intoindividual olefins by any method(s) known to one skilled in the art. Theindividual olefins can then be recovered and marketed. The BTX fractioncan be further separated into individual C₆ to C₈ . aromatic hydrocarbonfractions. Alternatively, the BTX fraction can further undergo one ormore reactions either before or after separation to individual C₆ to C₈hydrocarbons so as to increase the content of the most desired BTXaromatic hydrocarbon. Suitable examples of such subsequent C₆ to C₈aromatic hydrocarbon conversions are disproportionation of toluene (toform benzene and xylenes), transalkylation of benzene and xylenes (toform toluene), and isomerization of meta-xylene and/or ortho-xylene topara-xylene.

After the improved zeolite catalyst composition has been deactivated by,for example, coke deposition or feed poisons, to an extent that the feedconversion and/or the selectivity to the desired ratios of olefins toBTX has become unsatisfactory, the improved zeolite catalyst compositioncan be reactivated by any means or method(s) known to one skilled in theart such as, for example, calcining in air to burn off deposited cokeand other carbonaceous materials, such as oligomers or polymers,preferably at a temperature in the range of from about 400° C. to about1000° C. The optimal time periods of the calcining depend generally onthe types and amounts of deactivating deposits on the catalystcomposition and on the calcination temperatures. These optimal timeperiods can easily be determined by those possessing ordinary skill(s)in the art and are omitted herein for the interest of brevity.

The following examples are presented to further illustrate thisinvention and are not to be construed as unduly limiting its scope.

EXAMPLE I

This example illustrates the preparation of several catalysts which weresubsequently tested as catalysts in the conversion of a gasoline fluidsample to lower olefins (such as, ethylene and propylene) and aromatics(such as, BTX) as described in Example II.

Acid-Treated ZSM-5 Zeolite Catalyst

A commercially available ZSM-5 zeolite catalyst (provided by UnitedCatalysts Inc., Louisville, Ky., under product designation "T-4480"obtained as 1/16 inch extrudate) was treated with acid. To treat thecatalyst with acid, the catalyst was soaked in an aqueous hydrochloricacid (HCl) solution, having a concentration of 19 weight percent HCl(approximately 6N) for two hours at a constant temperature of about 90°C. After soaking, the catalyst was separated from the acid solution andthoroughly washed with water and dried. The acid-soaked, washed, anddried catalyst was calcined at a temperature of about 525° C. for 4hours.

Catalyst A (Control)

A 10.0 gram quantity of the above-described, acid-treated ZSM-5 zeolitecatalyst (commercially available "T-4480" treated with acid, asdescribed above) was impregnated, by an incipient wetness impregnationtechnique (i.e., essentially completely filling the pores of thesubstrate material with a solution of the incorporating elements), witha 12.5 gram quantity of an aqueous solution containing 12 weight percentof zinc nitrate hexahydrate (Zn(NO₃)₂ ·6H₂ O) and 32.8 weight percent ofchromium (III) nitrate nonahydrate (Cr(NO₃)₃ ·9H₂ O). This acid-treated,impregnated zeolite was then dried in air at 125° C. for 16 hours. Thethus dried, acid-treated, impregnated zeolite was then treated in asteam atmosphere for 6 hours at 650° C. with a H₂ O flow rate of 20mL/hr. The thus steamed zeolite was than calcined in a helium atmospherefor 2 hours at 538° C. A final product weighing 11.01 grams wasobtained. The final product contained a zinc (Zn) concentration of 2.995percent of the total weight of the final product. The final product alsocontained a chromium (Cr) concentration of 4.836 percent of the totalweight of the final product. The final product had an atomic ratio ofchromium to zinc of 2.032:1.

Catalyst B (Invention)

A 10 gram quantity of above-described, acid-treated ZSM-5 zeolitecatalyst (commercially available "T-4480" treated with acid, asdescribed above) was ion-exchanged, in a solution containing 0.50 gramsof zinc nitrate hexahydrate (Zn(NO₃)₂ ·6H₂ O), 1.35 grams of chromium(III) nitrate nonahydrate (Cr(NO₃)₃ ·9H₂ O), 13.45 grams of ammoniumnitrate (NH₄ NO₃), and 150 grams of deionized water, at a constanttemperature of 90° C. for 16 hours. The thus acid-treated, ion-exchangedzeolite was then washed with running water for about 30 minutes and thendried in air at room temperature (about 20° C. to about 25° C.) atatmospheric pressure (about 14.7 pounds per square inch absolute) forabout 2 hours. The thus dried, acid-treated, ion-exchanged zeolite wasthen treated in a steam atmosphere for 6 hours at 650° C. with a H₂ Oflow rate of 20 mL/hr. A final product weighing 9.72 grams was obtained.The final product contained a zinc (Zn) concentration of 1.131 percentof the total weight of the final product. The final product alsocontained a chromium (Cr) concentration of 1.806 percent of the totalweight of the final product. The final product had an atomic ratio ofchromium to zinc of 2:1 and a ratio of ammonium ions (NH₄ ⁺) to zincions (Zn⁺²) of 100:1.

EXAMPLE II

This example illustrates the use of the catalysts described in Example Ias catalysts in the conversion of a gasoline fluid to lower olefins(i.e., light olefins such as ethylene and propylene) and aromatics (suchas, BTX). The gasoline fluid sample had been produced in a commercialfluidized catalytic cracking unit (FCC).

For each of the test runs, a 5.0 g sample, of the catalyst materialsdescribed in Example I sized to 10-20 mesh, was placed into a stainlesssteel tube reactor (length: about 18 inches; inner diameter: about 0.5inch). Gasoline boiling range fluid from a catalytic cracking unit of arefinery was passed through the reactor at a flow rate of about 14mL/hour, at a temperature of about 600° C., and at atmospheric pressure(about 0 pounds per square inch gauge). The formed reaction productexited the reactor tube and passed through several ice-cooled traps. Theliquid portion remained in these traps and was weighed, whereas thevolume of the gaseous portion which exited the traps was measured in a"wet test meter". Liquid and gaseous product samples (collected athourly intervals) were analyzed by means of a gas chromatograph. Resultsof test runs for Catalyst A (Control) and Catalyst B (Invention) aresummarized in Table I. All test data were obtained after 8 hours onstream.

                  TABLE I                                                         ______________________________________                                                                   Sum                                                                           of                                                         BTX     Light Olefin*                                                                            BTX   Olefin/                                              Yield   Yield      and   BTX   Avg wt-%                               Catalyst                                                                              (wt-%)  (wt-%)     Olefin                                                                              Ratio Coke/hr**                              ______________________________________                                        A       41      19.8       60.8  0.48  1.0                                    (Control)                                                                     B       31      24.3       55.3  0.78  0.7                                    (Invention)                                                                   ______________________________________                                         *Ethylene and Propylene                                                       **Coke was determined at the end of the reaction by removing the catalyst     from the reactor and measuring the coke with a thermal gravimetric            analyzer (TGA), manufactured by TA Instruments, New Castle, Delaware.    

The test data presented in Table I clearly show that Invention CatalystB exhibited considerably less coking than Control Catalyst A. InventionCatalyst B also exhibited a remarkably improved (i.e ., greater)Olefin/BTX ratio when compared to Control Catalyst A. The performance ofInvention Catalyst B, as compared to Control Catalyst A, is superiorwhen comparing the light olefin yield and Olefin/BTX ratio. Theimprovement in catalyst performance is believed to be due to the novelprocess of making the invention catalyst by the novel process of a dualion-exchange of the ions of an acid-treated zeolite with ions of zincand at least one other metal followed by a steam treatment of suchacid-treated, ion-exchanged zeolite.

The difference in performance between the invention catalyst and thecontrol catalyst is certainly unexpected. One would not expect that dualion-exchange of an acid-treated zeolite by soaking such acid-treatedzeolite in a solution of, for example, zinc nitrate hexahydrate,chromium (III) nitrate nonahydrate, and ammonium nitrate, followed bysteam treatment, in lieu of incipient wetness impregnating theacid-treated zeolite with a solution of zinc nitrate hexahydrate andchromium (III) nitrate nonahydrate followed by steam treatment, wouldenhance the performance of the final zeolite catalyst composition. Theresults demonstrate that Invention Catalyst B, in which the acid-treatedzeolite is ion-exchanged and steam-treated, as opposed to impregnatedand steam-treated, gives a catalyst that is significantly superior toControl Catalyst A.

The results shown in the above example clearly demonstrate that thepresent invention is well adapted to carry out the objects and attainthe ends and advantages mentioned as well as those inherent therein.Reasonable variations, modifications, and adaptations can be made withinthe scope of the disclosure and the appended claims without departingfrom the scope of this invention.

What is claimed is:
 1. A process of making a catalyst for use inconverting hydrocarbons, said process consists essentially of:(a)treating a zeolite with an acid to form an acid-treated zeolite, (b)ion-exchanging said acid-treated zeolite with zinc and at least oneother metal selected from the group consisting of Group 6B elements ofthe periodic table of elements to form an acid-treated, ion-exchangedzeolite, and (c) steam treating said acid-treated, ion-exchanged zeoliteto form an acid-treated, ion-exchanged, steam-treated zeolite.
 2. Aprocess according to claim 1, wherein the amount of said zinc and saidat least one other metal ion-exchanged with said acid-treated zeolite isto provide an atomic ratio of said at least one other metal to said zincin said catalyst in the range of from about 0.1:1 to about 10:1.
 3. Aprocess according to claim 2, wherein the amount of said zincion-exchanged with said acid-treated zeolite is to provide aconcentration of said zinc in said catalyst in the range of from about0.1 weight percent of the total weight of said catalyst to about 10weight percent of the total weight of said catalyst.
 4. A processaccording to claim 3, wherein the amount of said at least one othermetal ion-exchanged with said acid-treated zeolite is to provide aconcentration of said at least one other metal in said catalyst in therange of from about 0.1 weight percent of the total weight of saidcatalyst to about 10 weight percent of the total weight of saidcatalyst.
 5. A process according to claim 4, wherein said ion-exchangingstep (b) further comprises ion-exchanging said acid-treated zeolite inan ion-exchange medium.
 6. A process according to claim 5, wherein saidion-exchange medium is selected from the group consisting of water,organic solvents, and combinations thereof.
 7. A process according toclaim 6, wherein said ion-exchange medium is water.
 8. A processaccording to claim 7, wherein said ion-exchange medium further comprisesan ammonium-containing compound, a zinc-containing compound and acompound containing said at least one other metal.
 9. A processaccording to claim 5, wherein said ion-exchange medium comprises anaqueous solution of an ammonium-containing compound, a zinc-containingcompound and a compound containing said at least one other metal.
 10. Aprocess according to claim 9, wherein said ammonium-containing compoundis selected from the group consisting of ammonium nitrate, ammoniumsulfate, ammonium chloride, ammonium bromide, ammonium fluoride, andcombinations thereof.
 11. A process according to claim 10, wherein saidammonium-containing compound is ammonium nitrate.
 12. A processaccording to claim 9, wherein said zinc-containing compound is selectedfrom the group consisting of zinc nitrate, hydrated zinc nitrate, zincacetate dehydrate, zinc acetylacetonate hydrate, zinc bromide, zinccarbonate hydroxide, zinc chloride, zinc cyclohexanebutyrate dihydrate,zinc 2-ethylhexanoate, zinc 2-ethylhexanoate, zinc fluoride, zincfluoride tetrahydrate, zinc hexafluoroacetylacetonate dihydrate, zinciodide, zinc molybdate, zinc naphthenate, zinc nitrate hexahydrate, zincperchlorate hexahydrate, zinc phosphate hydrate, zinc phthalocynine,zinc protoporphyrin, zinc selenide, zinc sulfate monohydrate, zincsulfide, zinc telluride, zinc tetrafluoroborate hydrate, zincmeso-tetraphenylprophine, zinc titanate, zinc trifluoromethanesulfonate,and combinations thereof.
 13. A process according to claim 12, whereinsaid zinc-containing compound is zinc nitrate hexahydrate.
 14. A processaccording to claim 9, wherein said compound containing said at least oneother metal is a compound selected from the group consisting ofchromium-containing compounds, molybdenum-containing compounds, andtungsten-containing compounds.
 15. A process according to claim 14,wherein said chromium-containing compound is selected from the groupconsisting of chromium(II) acetate, chromium(III) acetate, chromium(III)acetylacetonate, chromium(II) chloride, chromium(III) chloride,chromium(II) fluoride, chromium(III) fluoride, chromium(III) nitrate,hydrated chromium (III) nitrate, chromium (III) nitrate nonahydrate,chromium nitride, chromium(III) perchlorate, chromium(III) potassiumsulfate, chromium(III) sulfate, chromium(III) telluride, andcombinations thereof.
 16. A process according to claim 14, wherein saidmolybdenum-containing compound is selected from the group consisting ofmolybdenum(II) acetate, ammonium molybdate, ammonium dimolybdate,ammonium heptamolybdate, phosphomolybdic acid, molybdenum(III) bromide,molybdenum(II) chloride, molybdenum(IV) chloride, molybdenum(V)chloride, molybdenum(IV) sulfide, sodium molybdate, potassium molybdate,molybdenum fluoride, and combinations thereof.
 17. A process accordingto claim 14, wherein said tungsten-containing compound is selected fromthe group consisting of tungsten(V) bromide, tungsten(V) chloride,tungsten(VI) chloride, tungsten(IV) sulfide, tungstic acid, andcombinations thereof.
 18. A process according to claim 9, wherein saidcompound containing at least one other metal is chromium (III) nitratenonahydrate.
 19. A process according to claim 9, wherein theconcentration of said acid-treated zeolite in said solution is in therange of from about 0.01 gram of said acid-treated zeolite per liter ofsaid solution to about 200 grams of said acid-treated zeolite per literof said solution.
 20. A process according to claim 9, wherein theconcentration of said ammonium-containing compound in said solution isin the range of from about 0.1 gram of said ammonium-containing compoundper liter of said solution to about 500 grams of saidammonium-containing compound per liter of said solution.
 21. A processaccording to claim 9, wherein the concentration of said zinc-containingcompound in said solution is in the range of from about 0.1 gram of saidzinc-containing compound per liter of said solution to about 500 gramsof said zinc-containing compound per liter of said solution.
 22. Aprocess according to claim 9, wherein the concentration of said compoundcontaining said at least one other metal in said solution is in therange of from about 0.1 gram of said compound containing sad at leastone other metal per liter of said solution to about 500 grams of saidcompound containing said at least one other metal per liter of saidsolution.
 23. A process according to claim 9, wherein said solution canbe subjected to a temperature in the range of from about 30° C. to about200° C. for a time period in the range of from about 1 hour to about 100hours under a pressure in the range of from about atmospheric to about150 pounds per square inch absolute.
 24. A process according to claim 1,wherein said treating step (a) comprises contacting said zeolite withsaid acid selected from the group consisting of sulfuric acid,hydrochloric acid, nitric acid, phosphoric acid, formic acid, aceticacid, trifluoroacetic acid, trichloroacetic acid, p-toluenesulfonicacid, methanesulfonic acid, partially neutralized acids and combinationsthereof.
 25. A process according to claim 24, wherein said acid ishydrochloric acid.
 26. A process according to claim 1, wherein saidzeolite is selected from the group consisting of ZSM-5, ZSM-8, ZSM-11,ZSM-12, ZSM-35, ZSM-38, and combinations thereof.
 27. A processaccording to claim 26, wherein said zeolite is ZSM-5.
 28. A processaccording to claim 1, wherein said steam treating step (c) comprisesexposing said acid-treated, ion-exchanged zeolite to a steam atmospherehaving a concentration of steam exceeding about 90 molar percent,apressure in the range of from about atmospheric to about 1000 pounds persquare inch absolute, a temperature in the range of from about 100° C.to about 1000° C., and a time period in the range of from about 0.1 hourto about 30 hours.